Fe/Cu diatomic catalysts for electrochemical nitrate reduction to ammonia

Electrochemical conversion of nitrate to ammonia offers an efficient approach to reducing nitrate pollutants and a potential technology for low-temperature and low-pressure ammonia synthesis. However, the process is limited by multiple competing reactions and NO3− adsorption on cathode surfaces. Here, we report a Fe/Cu diatomic catalyst on holey nitrogen-doped graphene which exhibits high catalytic activities and selectivity for ammonia production. The catalyst enables a maximum ammonia Faradaic efficiency of 92.51% (−0.3 V(RHE)) and a high NH3 yield rate of 1.08 mmol h−1 mg−1 (at − 0.5 V(RHE)). Computational and theoretical analysis reveals that a relatively strong interaction between NO3− and Fe/Cu promotes the adsorption and discharge of NO3− anions. Nitrogen-oxygen bonds are also shown to be weakened due to the existence of hetero-atomic dual sites which lowers the overall reaction barriers. The dual-site and hetero-atom strategy in this work provides a flexible design for further catalyst development and expands the electrocatalytic techniques for nitrate reduction and ammonia synthesis.

Detection of NO3 − . The concentration of NO3 − was determined at different voltages using the ultravioletvisible (UV-Vis) spectrophotometry. Each voltage was hold for 0.5 h in 1 M KOH and 0.1 M KNO3 before nitrogen quantification. After that, 1.0 mL electrolyte was removed out of the electrolytic cell and diluted to 5 mL. 0.1 mL HCl (1 M) and 0.01 mL sulfamic acid (Shanghai Macklin Biochemical Technology Co., Ltd., AR, 99.5%, 0.8 wt%) were further added in the solution. After 15 minutes, the UV-vis absorption spectra were recorded with a Shimadzu UV-3600 plus spectrophotometer. The total absorbance of NO3 − was calculated by the following equation: A=A220−2*A275 (where A220 and A275 are the absorbance coefficients at 220 nm and 275 nm, respectively) 1 . The standard curve can be made by measuring the UV-vis spectra of varied concentrations of KNO3 solutions.
Detection of NO2 − . The nitrite concentration was measured by UV-vis spectrophotometry according to the standard method. Firstly, the colour reagent was prepared by mixing sulfonamide (Shanghai Macklin Biochemical Technology Co., Ltd., AR, 99.5%, 4 g), N-(1-naphthyl) ethylenediamine dihydrochloride (Shanghai Macklin Biochemical Technology Co., Ltd., >98%, 0.2 g), phosphoric acid (H3PO4, Sinopharm Chemical Reagents Co., Ltd., GR, ≥85.0%, 10 mL, ρ=1.685 g mL -1 ), and deionized water (50 ml). The electrolyte sample should be diluted to the detection range. Then 0.1 mL of the colour reagent was mixed with 5 mL of the sample solution and rested for 20 min at room condition. The absorption intensity at a wavelength of 540 nm was recorded by UV-Vis absorption spectrum. The concentration-absorbance curve was linear fitted using a series of standard KNO2 (potassium nitrite, Aladdin, AR, 97%) solutions by the same processes. The concentration of NO2 − product was calculated based on the calibrated curve.
Faradaic efficiency and yield of NH3. The faradaic efficiency (FE) of NH3 production was determined by the following equation: Where F is Faraday constant (96485 C mol −1 ), CNH3 is the concentration of NH3 (μg mL −1 ) in the electrolyte, V is the volume of the electrolyte, Q is the charge consumed for NH3 generation.
The yield rate (YR, mgNH3 h −1 cm −2 ) of NH3 can be calculated using the following equation: Where t is the electrolysis time; A is the geometric area of the electrode (1 cm −2 ).
Energy consumption of NH3 production. Assuming the overpotential of anodic electrode (the water oxidation) is zero, the half-cell energy efficiency (EE) defined as the ratio of chemical energy to applied electrical power was calculated with the following equation: where ENH3 0 is the equilibrium potential (0.69 V) of nitrate electroreduction to ammonia in alkaline media; FE(NH3) is the faradaic efficiency for NH3; 1.23 V is the equilibrium potential of water oxidation (i.e. assuming the overpotential of the water oxidation is zero); E is the applied potential (vs. RHE) for NH3 production. Energy consumption (EC, Wh gNH3 −1 ) was calculated by EC = The vacuum thickness was set to be 15 Å to minimize interlayer interactions. The solvation effect was not included since the ignorable energy change was witnessed. VASPKIT was adopted to obtain the DOS diagrams 7 . Wannier orbitals were calculated using a wannier90 code 8 .
Electrochemical nitrate reduction pathway. Based on computational hydrogen electrode (CHE) model 9 , the Gibbs free energy (ΔG) calculations of each elementary step can be determined as: where ΔE is the energy obtained from DFT calculations. ΔEZPE and ΔS are the correction of zeropoint energy and entropy, respectively. T is temperature (298.15 K). ΔU and ΔpH represent the effect of voltage and pH, respectively.
To avoid calculate the free energy of charged NO3 − directly, gaseous HNO3 is chosen as a reference instead 10 . Following the method of previous report, the adsorption energy of NO3 − (ΔG*NO3) could be determined by: where G*NO3, G*, GHNO3(g), and GH2(g) are the Gibbs free energy of NO3 adsorbed on substrate, HNO3, and H2 molecules in the gas phase, respectively. ΔGcorrect denotes the correction of adsorption energy and is set to 0.392 eV. Electrochemical reduction from nitrate to NH3 involves nine protons and eight electrons. The whole reaction can be summarized as: FEs decreased dramatically at the more negative potential owing to the competitive HER. In general, the catalyst presented the appreciable ammonia yield rates and high appreciable selectivity under different nitrate concentrations, demonstrating the high activity of Fe/Cu-HNG.
Supplementary Fig. 30 The schematic illustration of customized electrochemically cell.
Supplementary Fig. 31 Full reaction paths for NO3reduction reaction.
Supplementary Fig. 32 (a, b) The crystallographic model and atomic arrangement for Fe/Cu-HNG.
After optimization, the distance between two adjacent Fe and Cu atoms is around 2.26 Å, which is close to the observed separation from the STEM image in Fig. 2c.
In the local structure of N2Fe-CuN2, each metal atom is triply coordinated with two nitrogen atoms and one metal (similar to a Y-type ML3 coordination). The 4sp 2 hybrid orbitals of Fe form three in-plane bonds. The Fe orbital with some mixing and $ orbital with some mixing could also contribute to the bonding interaction with 2 N and Cu while the Fe orbital remains weakly bonding with them. The Fe and orbitals weakly bond with 2 N , and may also form weak d-d and d-d δ interactions with Cu, respectively.